Display devices such as televisions, etc. has an OSD (On-Screen-Display) function, so as to display a menu screen thereon for setting. Moreover, such display devices are configured to be able to display subtitles, data broadcast or the like apart from the menu display, by using the OSD function. Hereinafter, a display operation for display a certain screen by using the OSD function is referred to as an OSD display operation. The OSD display operation is carried out such that a 2-dimensional plane region (hereinafter, referred to as an OSD plane) for image formation for a menu screen, subtitle, or the like is set, and the OSD plane is outputted on a display section of the display device.
Moreover, in the OSD display, pixel colors in image data are generally expressed based on a RGB expression or a YCbCr expression. In this case, however, such problems should be encountered that storing the pixel colors expressed on the RGB expression or the YCbCr expression requires the memory to store a large data amount to occupy a memory space, updating the OSD display requires a high read/write speed to perform reading and writing to the memory, that is, requires a wider bandwidth for memory. In order to encounter this problems, a technique in which the pixel colors are expressed based on an index color expression is adopted. That is, in general, the display devices expresses colors by combinations of intensities of RGB. However, when the index color expression is used, only necessary one or ones of the combinations are selected and indexed, so that the pixel colors are indicated by color codes. Moreover, in order to display or convert the pixel colors, a look-up table (pallet (CLUT)) between the color codes and the colors of the RGB expression is referred to allot actual colors to the pixels.
In some cases, it is demanded that OSD planes for different kinds of usages should be displayed in combination with each other in superimposition with an image. In such cases, a plurality of OSD planes are required. How many OSD planes can be displayed is dependent on such hardware restrictions as a storage capacity of the memory to store the image data for image formation in the OSD planes, read/write speed of OSD display data to the memory, that is, bandwidth etc. of the memory. As such, the number of the OSD planes to be displayed is restricted. In case where a plurality of OSD planes are superimposed and thereby combined with each other, the following problems are encountered when such a composite OSD plane prepared by combining the OSD planes is displayed.
For example, as illustrated in FIG. 2, there is a case where a composite product (composite screen) synthesized by partially imposing OSD planes U and L expressed in the index color expression has an superimposed portion F at which the OSD planes U and L are superimposed. If the composite screen is also expressed in the index color expression, the superimposed portion F sometimes has a color that does not exist on a pallet for the composite screen. In such a case, a color of one of the OSD planes U and L which is superimposed over the other is adopted as the color of the portion F (in FIG. 2, the color of the OSD plane U is adopted). That is, there is a problem that the superimposition of displays in the OSD planes U and L cannot be precisely expressed. Note that FIG. 2 is a view of a conventional art and illustrates an example of a composite screen synthesized by superimposing the different OSD planes U and L expressed in the index color expression.
In order to appropriately express the color of the superimposed portion F, it can be considered to synthesize a composite product by combining the OSD planes U and L converted from the index color expression to the RGB expression, as illustrated in FIG. 3. Note that FIG. 3 is a view of a conventional art and illustrates a composite product synthesized by superimposing the different OSD planes U and L expressed in the index color expression and converted to the RGB expression. However, in this case, the composite screen should be stored in the RGB expression, thereby requiring the memory or the like to store a larger data amount than storing the composite screen in the index color expression as illustrated in FIG. 2.
Moreover, the OSD plane can be used not only for displaying the aforementioned display contents but also for giving a fading effect to an image displayed on the display device. The fading effect may be a fading-out process for darkening an image gradually to black and a fade-in process for appearing an image gradually from a black display. In general, such a fading effect is realized by specially using an OSD plane colored in a particular color (usually black) for the fading effect (see Patent Literature 1, for example). In a broadcast receiving device disclosed in Patent Literature 1, fading out is carried out by gradually increasing transmittance of a black OSD plane from 0% to 100%. On the contrary, fading in is carried out by gradually decreasing the transmittance of the black OSD plane from 100% to 0%.
Moreover, Patent Literature 2 discloses an electronic circuit capable of combining various OSD images (OSD planes) with a source image (image). The electronic circuits causes a memory to store therein (i) the source image, (ii) index color data indicative of color codes of pixels constituting the OSD image, (iii) pallet data including image signal values and α values for indicating composition ratios. By an image processing section of the electronic circuit, a color for each pixel is determined by combining, at a composition ratio determined by an α value, the source image data with an image signal value indicated by the color code. By this, the electronic circuit can change a transmittance of an OSD image by updating the α values, thereby being able to synthesize various OSD images. It is also possible that different OSD planes are combined and displayed in combination, where one of the different OSD planes is utilized to attain such a display effect, such as fading effect, that a display is dynamically changed over a certain time period. If the composite screen of such a display is stored in the index color expression, the superimposed portion F of the composite screen would possibly fail to appropriately reflect the dynamic display change. For example, in case of the one of the OSD planes that is mono-colored and whose transmittance is changed, a color of the other one of the OSD planes cannot be appropriate if, in the superimposed portion F the color of the one of the OSD planes as a result of the change in the transmittance does not exist on the pallet of the composite screen. Thus, in case where the composite screen is stored in the index color expression, there is a possibility that the composite screen thus stored does not reflect the dynamic display change appropriately. Moreover, it can be considered to express and store the composite screen in the RGB expression. In this case, as described above, the composite screen can reflect the dynamic display change appropriately, but results in an increase in data amount to display the composite screen.